1. Field of the Invention:
This invention relates to a driver circuit for a D.C. motor which circuit includes a semiconductor integrated circuit.
2. Description of the Related Art
In general, a circuit for controlling a D.C. motor comprises an amplifier circuit for amplifying pole detection signals of a rotor such as from Hall elements, a waveform composing circuit for forming a predetermined driving control signal from the amplified pole detection signal, and a driver circuit for driving stator coils based on the driving control signal.
FIG. 3 shows a typical conventional driver circuit for driving the coil. This driver circuit is a circuit for a three-phase D.C. motor. La, Lb, Lc in FIG. 3 designate respective stator coils; Q1a, Q2a, inflow and outflow output transistors for driving the coil La; Q1b, Q2b, inflow and outflow output transistors for driving the coil Lb; and Q1c, Q2c, inflow and outflow output transistors for driving the coil Lc. The transistors Q1a, Q1b, Q1c located at the power-source side of each transistor output circuit are output transistors forming respective current inflow paths, while the transistors Q2a, Q2b, Q2c located at the ground side of each transistor output circuit are output transistors forming respective current outflow paths. Now assuming that current is flowing in the coils La, Lb, Lc in the direction of arrows, the current from the transistor Q1b flows in the coil Lb, and the current from the transistor Q1c flows in the coil Lc, and their composite current flows out of the transistor Q2a via the coil La.
FIG. 2 shows a conventional driver circuit per one phase for driving the coil of such D.C. motor. Q1 in FIG. 2 designates an output transistor at the current inflow side; Q2, an output transistor at the current outflow side; and Q10, an upstream-stage transistor for driving the output transistor of the current inflow side. Generally, such conventional driver circuit, along with the pole-detection-signal amplifier circuit and the waveform composing circuit, is integrated in a single semiconductor integrated circuit; in which case a p-n-p transistor is low in allowable maximal current per unit area, compared to an n-p-n transistor. To this end, as shown in FIG. 2, an n-p-n transistor is used also for the current inflow output transistor Q1, and in order to invert the driving control signal, a p-n-p transistor Q10 is located upstream of the n-p-n transistor to drive the current inflow output transistor Q1.
However, with the conventional driver circuit of FIG. 2, the current amplification rate (h.sub.FE) would vary widely depending on the type, i.e. p-n-p or n-p-n, of transistor. Thus, even though there is no difference between the currents Il, I2 flowing in the circuit connected to the upstream part of this driver circuit, the ratio of the driving currents Iol, Io2 would not be constant.
As a consequence, currents flowing in the respective coils would be unbalance to cause ripple variation of torque and also reduction of torque. If the base current I2 of the current outflow output transistor Q2 is adequately large, the current flowing in the coil will be determined the current value of the current inflow output transistor of the other phase. Therefore fluctuation of the coil current is somewhat improved. In this case, since each of the current inflow output transistor Q1 and the current outflow output transistor Q2 cannot be shifted to the ON state smoothly, yet an adequate driving characteristic is difficult to achieve.